A communication method as a successor of the W-CDMA (Wideband-Code Division Multiple Access) scheme or the HSDPA (High Speed Downlink Packet Access) scheme, namely, the LTE (Long Term Evolution) scheme has been considered by the W-CDMA standardization organization 3GPP, and the specification setting work is underway.
As a radio access method of the LTE scheme, use of the OFDMA in the downlink and the SC-FDMA (Single-Carrier Frequency Division Multiple Access) in the uplink is under consideration.
The OFDMA is a method for dividing a frequency band into plural narrow frequency bands (sub-carriers) and transmitting data loaded on the respective divided frequency bands. In this method, high-speed transmission is achieved and frequency utilization efficiency is improved by arranging sub-carriers densely on the frequency bands in such a manner that the sub-carriers partially overlap but do not interfere with each other.
The SC-FDMA is a transmission method which reduces interference between terminals by dividing a frequency band and transmitting data by using frequency bands different among plural terminals. The SC-FDMA has a feature of less fluctuation in the transmission power, which achieves low power consumption and wide coverage of terminals.
The LTE scheme is a system in which plural mobile stations communicate with each other by sharing one or more physical channels in both uplink and downlink.
A channel shared by plural mobile stations is generally called a shared channel, which is, in the LTE system, a “Physical Uplink Shared Channel (PUSCH)” in the uplink and a “Physical Downlink Shared Channel (PDSCH)” in the downlink.
Also, such a shared channel is, as a transport channel, an “Uplink Shared Channel (UL-SCH)” in the uplink and a “Downlink Shared Channel (DL-SCH)” in the downlink.
In such a communication system using shared channels described above, it is necessary to select which mobile station UE the shared channel is to be assigned to, and to signal to the selected mobile station UE that the shared channel is assigned thereto, for each sub-frame (1 ms, in the LTE scheme).
In the LTE scheme, a control channel used for the signaling is called the “Physical Downlink Control Channel (PDCCH)” or “Downlink L1/L2 Control Channel (DL L1/L2 Control Channel)”.
Meanwhile, the processing for each sub-frame of selecting which mobile station UE the shared channel is to be assigned to is generally called the “scheduling”.
In this case, the processing may also be called the “dynamic scheduling”, because the mobile station UE to which the shared channel is assigned is dynamically selected for each sub-frame.
Furthermore, the expression “assigning the shared channel” described above may be alternatively expressed as “assigning a radio resource for the shared channel”.
Information of the physical downlink control channel includes, for example, the “downlink scheduling information”, the “uplink scheduling grant”, and the like.
The “downlink scheduling information” includes, for example, regarding the downlink shared channel, downlink resource block assignment information, UE-IDs, the number of streams, information on the precoding vector, data size, modulation method, information on the HARQ (hybrid automatic repeat request), and the like.
Meanwhile, the “uplink scheduling grant” includes, for example, regarding the uplink shared channel, uplink resource block assignment information, UE-IDs, data size, demodulation method, uplink transmission power information, information on demodulation reference signal in the uplink MIMO, and the like.
Note that, the “downlink scheduling information” and the “uplink scheduling grant” described above may be collectively referred to as “downlink control information (DCI)”.
Note that, a mobile station uses a “UE-ID (RNTI)” in the uplink scheduling grant or the downlink scheduling to identify whether or not the uplink scheduling grant or the downlink scheduling is transmitted to the mobile station itself.
More specifically, CRC bits included in the uplink scheduling grant or the downlink scheduling are masked by the RNTI of the mobile station of the transmission destination.
The mobile station performs a CRC check by use of the CRC bits. Then, when the result of the CRC check is OK, the mobile station determines that the uplink scheduling grant or the downlink scheduling is transmitted to the mobile station itself. When the result of the CRC check is not OK, the mobile station determines that the uplink scheduling grant or the downlink scheduling is not transmitted to the mobile station itself.
Note that, the CRC bits are bits used for determining whether a transmitted signal is wrongly decoded or correctly decoded.
Accordingly, when a certain mobile station receives a signal in which the CRC bits are masked by the RNTI of another mobile station, the result of the CRC check becomes not OK even if the signal is actually received without an error.
In addition, the number of bits for the CRC bits and the RNTI is 16 bits, for example.
Note that, a mobile station generally attempts to decode 40 uplink scheduling grants or 40 pieces of downlink scheduling information, for example, in a single sub-frame.
In this case, the approximately 40 uplink scheduling grants or the 40 pieces of downlink scheduling information include a signal actually transmitted to the mobile station itself, a signal transmitted to another mobile station, a signal including only noise without any transmission signal, and the like, for example.
On one hand, in “Persistent scheduling”, which has been studied for the purpose of implementing VoIP and the like, a radio base station eNB is configured to persistently assign an uplink or downlink radio resource (PUSCH or PDSCH) to the mobile station, in a predetermined cycle, starting at a sub-frame (assignment starting time) specified by a PDCCH (uplink scheduling grant or downlink scheduling information piece). Note that, the “Persistent scheduling” may be called “Semi-Persistent scheduling”.
Here, the assignment starting time is a sub-frame in which the downlink scheduling information is transmitted in the case of downlink, and is an uplink transmission sub-frame specified by the uplink scheduling grant in the case of uplink. In addition, the predetermined cycle is 20 ms, for example.
In the “Persistent scheduling”, the uplink scheduling grant or the downlink scheduling information is transmitted via a PDCCH only for the initial transmission, and no uplink scheduling grant or no downlink scheduling information is transmitted via the PDCCH for the transmissions after the initial transmission. Thus, the radio resources (overhead) required for transmitting the uplink scheduling grants or the downlink scheduling information can be reduced. As a result, efficient communications can be performed.
Note that, as described above, there are two types of the uplink scheduling grant or the downlink scheduling grant information. One of the types is for notifying, to the mobile station, a resource assignment by the Dynamic scheduling. The other one of the types is for notifying, to the mobile station, a resource assignment by the Persistent scheduling.
In this case, whether the assignment is by the Dynamic scheduling or by the Persistent scheduling described above is identified on the basis of an RNTI and the like, for example, the RNTI set in the uplink scheduling grant or the downlink scheduling information.
More specifically, an RNTI for the Dynamic scheduling and an RNTI for the Persistent Scheduling are defined, and whether the assignment is by the Dynamic scheduling or by the Persistent Scheduling is identified on the basis of the result of the CRC check of the corresponding RNTI.